Could Arctic Sea Ice Retreat Drive Changes in Air Pollution Levels?
The Arctic is often thought of as a remote region, lacking in large anthropogenic emission sources. However, studies have shown that there is substantial transport of air pollution to northern polar latitudes from North America and, more importantly, Eurasia. Also, the opening of new shipping routes through the northern passages in the future could lead to substantial increases in the amount of pollutants present in the Arctic.
Another major characteristic of the Arctic is that it is warming faster than the rest of the globe. As a result, there has been a significant reduction in year-round and especially summer Arctic sea ice in recent years. Models show that there is a large possibility for further reductions in a warmer climate, with the Arctic projected to become near ice-free during summer later this century.
This sea ice retreat has significant effects on high-latitude ecosystems and on the evolution of climate change itself, through the change of Earth surface's reflectivity. But could it be important for atmospheric chemistry? In a new study, we examined how Arctic sea ice retreat could impact oxidation and tropospheric ozone chemistry. Two potential effects were investigated:
• The reduction of sea ice causes the surface reflectivity to strongly decrease in the Arctic, which leads to decreases in shortwave radiation in the atmosphere and, thus, lower photo-dissociation rates of tropospheric gases. Photo-dissociation of the ozone molecule is the major process that leads to the production of OH (hydroxyl radical), the main oxidizing (i.e., cleansing) gas species in the troposphere. Therefore, we expect that a decrease in photo-dissociation rates would lead to a decrease in OH concentrations and a weakening of the oxidizing capacity of the Arctic troposphere.
• Another important feature of Arctic chemistry is the involvement of bromine gas species in sudden ozone depletion events in the lower troposphere during spring. Satellite remote sensing has indicated that one possible mechanism leading to these events is sea-salt aerosol production from snow lying on sea ice during blowing snow events and subsequent bromine release ("bromine explosions"). Reactive bromine species are released from inert salt ions through a process that is not completely understood yet. In an extreme case with Arctic sea ice retreating even in spring, this process could become far less important.
Our analysis was based on sensitivity experiments using a tropospheric chemistry computer model. The changes in atmospheric oxidants (OH and ozone) were examined in a scenario where Arctic sea ice was removed for late summer ("realistic scenario") and in a scenario where it was removed for the whole summer and spring ("extreme scenario").
In the "realistic scenario", we found that late-summer OH concentrations over northern high latitudes were strongly reduced (30-60%; see Figure 2) when removing sea ice in that season, due to the reduction in photo-dissociation rates. The effect on ozone (driven by the effects on OH) is smaller but detectable (reductions of 0-5%). These results suggest that the tropospheric oxidizing capacity could change dramatically over the Arctic if summer sea ice is to retreat in the future, something that could impact the removal of important gases (methane, carbon monoxide) in this region.
On the other hand, in the "extreme" scenario (Figure 3), where sea ice is removed both for summer and spring, we found large changes in both OH (reductions of up to 100%) and ozone (increases of up to 50-60%). The effect on OH, again caused by the lower photo-dissociation rates, is not important as it is associated with very low OH concentrations at high northern latitudes in spring. But the effect on ozone, which is caused by the reduction of bromine explosions due to the sea ice removal, is particularly important. Even significantly populated high-latitude regions (Northern Europe, Canada, Japan etc) experience ozone increases of up to 20%.
We note that this study has not taken into account the potential changes in atmospheric humidity caused by sea ice removal, which should be a focus of future work. Nevertheless, it suggests that future sea ice reduction could lead to possibly important changes in oxidation over the Arctic, which could have implications for regions outside of the Arctic, under specific conditions. This can be a critical issue, since the northern high-latitude regions not only include very important ecosystems, but they could also face a substantial increase of human population in a changing world with a warming climate.
Voulgarakis, A., X. Yang, and J. A. Pyle, 2009: How different would tropospheric oxidation be over an ice-free Arctic? Geophys. Res. Lett., 36, L23807, doi:10.1029/2009GL040541.
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